A semiconductor device in the related art proposed as, for example, a switching element used in an inverter has an IGBT region where an IGBT element is provided and an FWD region where an FWD element is provided (see, for example, Patent Literature 1).
More specifically, the proposed semiconductor device includes a base layer in a surface-layer portion of a semiconductor substrate forming an N−-type drift layer, and multiple trenches provided to penetrate through the base layer. Each trench is provided with a gate insulating film to cover a wall surface and a gate electrode is provided on the gate insulating film.
A P-type collector layer and an N-type cathode layer are provided on a back surface side of the semiconductor substrate. An N+-type emitter region is provided to the base layer in a portion located above the collector layer. An upper electrode to be electrically connected to the base layer and the emitter region is provided on a surface side of the semiconductor substrate, and a lower electrode to be electrically connected to the collector layer and the cathode layer is provided on the back surface side of the semiconductor substrate. A region where the collector layer is provided and a region where the cathode layer is provided on the back surface side of the semiconductor substrate are referred to as an IGBT region and an FWD region, respectively. When configured as above, the N-type cathode layer and drift layer and the P-type base layer form an FWD element having an PN-junction in the FWD region of the semiconductor substrate.
In the semiconductor device configured as above, when a voltage lower than a voltage applied to the lower electrode is applied to the upper electrode and a turn-on voltage is applied to the gate electrode, an N-type inversion layer (that is, a channel) is formed in the base layer in a portion in contact with the trenches. Hence, electrons are supplied to the drift layer from the emitter region via the inversion layer and holes are supplied to the drift layer from the collector layer. Eventually, a resistance value of the drift layer decreases due to conductivity modulation. The IGBT element thus switches ON. The turn-on voltage referred to herein means a voltage which raises a gate-emitter voltage Vge above a threshold voltage Vth of an insulated gate structure.
The FWD element switches ON when a voltage higher than a voltage applied to the lower electrode is applied to the upper electrode and a voltage between the upper electrode and the lower electrode rises above a forward voltage. Holes are thus injected into the drift layer from the base layer.
In the semiconductor device configured as above, however, the base layer is common between the IGBT region and the FWD region. Hence, an impurity concentration of the base layer in the FWD region (that is, an anode layer) becomes too high. In such a case, when the FWD element switches ON (performs a diode operation), too many holes are injected, which results in a problem that a reverse recovery charge becomes so high that a recovery current is increased.